The tailoring of material surface properties, such as affinities towards bulk materials, is highly desirable in many applications, in particular in emerging healthcare and microelectronics applications, and involves issues such as colloid stabilization, dewetting and adhesion. In the field of human implants, for instance, there is a growing interest and need for surface modification strategies for controlling the biological interaction between cells or tissues and a device surface.
One way of controlling the surface properties of materials is to deposit a polymer brush thereon. Polymer brushes are created by attaching (grafting) polymers by one end of their chain to a surface at a sufficiently high density so that the chains, with respect to their preferred configuration, substantially stretch away from the surface in order to avoid overlapping of chains. Methods of end-grafting (tethering) polymer chains on surfaces are increasingly being investigated and utilized for modifying the surface properties of materials.
Although variation in the chemical composition of the individual polymers may be used to expand the range of properties of a polymer brush of tethered polymer chains, the properties of the brush mainly depend on the thickness of the brush layer and the grafting density at the material surface. Whereas the thickness of the brush is known to depend strongly on the length of the polymer, the brush grafting density at the material surface is less easy to control and depends strongly on the method used to form the polymer brushes.
End-grafted polymer brushes have been successfully prepared by any of four different techniques:
(1) “grafting to” involves the chemisorption from solution or onto a surface of pre-formed, (mono)end-functionalized polymer chains to form a tethered polymer brush,
(2) “grafting from” involves the de-novo generation of the individual polymer chains by monomer polymerization directly at the surface in situ, for example, by using a self-assembled monolayer (SAM) of initiators covalently bonded to the surface as starting points of a radical or anionic polymerization process.
(3) by physisorption of hydrophobically modified polyelectrolyte block copolymers, and
(4) by a Langmuir-Blodgett (LB) technique using polymer-based amphiphile.
Whereas the latter two techniques involve physisorption of the brush polymer chains to the surface, the covalent bonding in the “grafting to” or “grafting from” approach generally results in a more stable attachment of the brushes to the surface.
One problem with brushes obtained with the “grafting to” approach is that they generally have a low grafting density and limited thickness because, during the grafting process, initially chemisorbed polymer chains on the surface may shield new incoming chains from accessing the underlying surface due to a variety of intermolecular repulsive interactions (e.g., steric, electrostatic). Therefore, in producing high-density brushes, the “grafting to” procedure has proven to be experimentally much more challenging than the “grafting from” procedure.
The “grafting from” procedure, wherein initiators are immobilized on the surface and polymerization of monomers is initiated therefrom, generally allows for a fairly good control over the production of brushes with large thickness since the length of the polymer (e.g. the number of repeat units in the polymer) may for instance be controlled by adjusting the polymerization time or monomer concentration during growth of the polymer. The “grafting from” procedure also offers a fairly good control over the grafting density of the polymer brush—in the ideal case the grafting density is simply equal to the surface density of the polymerization initiators. While a further advantage of such methods is their ease of use and the ability to prepare brushes from a variety of monomers, a major disadvantage of the “grafting from” procedure is the hard-to-control polymerization process, which usually leads to brushes with broad molecular weight distributions. The “grafting from” procedure is a chain polymerization reaction in which the growth of a polymer chain proceeds exclusively by reactions between monomers and reactive sites on the polymer chain whereby at the end of each growth step reactive sites are regenerated. However, during such reactions, chain transfer usually occurs in which the activity of the kinetic-chain carrier is transferred from the growing macromolecule or oligomer molecule to another molecule or another part of the same molecule, leading to chains of different length, and a concomitant broad molecular weight distribution or high “polydispersity” of the brush.
Surface-initiated “living” radical polymerization procedures, such as the atom transfer radical polymerization (ATRP) procedure (e.g. U.S. Pat. No. 6,407,187), which are by definition free from chain transfer and chain termination, provides in principle for a method that yields polymers with a molecular weight distribution Mw/Mn of <1.5. For many applications, this is still considered as a high polydispersity value. Moreover, the chain length of the brush is not very well defined and the grafting density is not controllable.